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B Question about friction -- car accelerating so fast its front wheels rise off the pavement

  1. Jul 7, 2016 #1
    Imagine a car race about to take place. A car is starting at rest and when the timer goes off, the driver steps on the gas. The car starts off so fast, that its front wheels rise upwards so that all the weight of the car is held up by the 2 rear wheels. The car travels this way for a few seconds.

    During the time which the car is on its two rear wheels, is the force of friction from the ground assisting the car's forward movement or opposing it?
     
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  3. Jul 7, 2016 #2

    Doc Al

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    It's the forward acting friction on the rear tires that gives the car its acceleration and raises the car's front wheels.
     
  4. Jul 7, 2016 #3
    "Forward acting" friction is quite new to me, since usually most problems I've done include friction forces that are opposing the forward movement. But it makes sense, if one was on friction-less ice, one would need friction to move anywhere. Just quite hard to wrap my head around it.
     
  5. Jul 7, 2016 #4

    Doc Al

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    Yes, many associate friction with opposing motion, but this is a good example why that is a mistake. It's friction that accelerates the car when you step on the gas. (And, of course, friction also slows the car when you step on the brake.)
     
  6. Jul 7, 2016 #5

    russ_watters

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    Think about the direction the wheels are turning (and maybe think about what it is like if they lose grip and spin in place) the tread is moving backwards across the ground, applying a force to the ground pushing backwards. The ground applies an equal and opposite force pushing the car forward.
     
  7. Jul 7, 2016 #6

    sophiecentaur

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    I agree. It's a case where just learning an (over-) simple rule without understanding can lead you into misconceptions. All forms of traction involve a friction type force acting in the 'wrong' direction. Clearly friction doesn't always involve resisting motion in a simple sense. Of course, the friction on the road 'resists' the wheel rotating freely and that is what produces a reaction force to drive the car forward.
     
  8. Jul 7, 2016 #7

    sophiecentaur

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    Not all the force from the wheels is available for accelerating the car. There are rolling losses on the wheel - due to deformation of the tyres and even the road surface. The leading edge of the tyre is constantly being compressed / flattened as it makes contact with the road and the resilience at the back edge doesn't compensate for this because of the hysteresis during the deformation. That uses up motive power so it is the equivalent of a force acting against the car's motion . This is the Rolling Friction.
     
  9. Jul 8, 2016 #8
    Just to verify this, what raises the cars front wheels relative to ground, is that the torque is unbalanced, the torque from the front wheels overcomes the torque from the weight (taken both as to point of contact of the rear wheel). Here is my dump question, why we don't see a car raising its rear wheels ( I guess the car has to go with the reverse and with big acceleration , but reverse doesn't provide big acceleration in most cars right?).
     
    Last edited: Jul 8, 2016
  10. Jul 8, 2016 #9

    Svein

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    As long as the power from the engine is applied to the rear wheels - if they lift from the ground, the acceleration force would instantly disappear...
     
  11. Jul 8, 2016 #10

    sophiecentaur

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    Irrespective of any drive to the front wheels, the front wheels will lift when the torque from the back wheels is greater than the torque due to the weight of the car and the distance of the CM from the rear axle.
    That bald statement ignores any sharing of available torque from the engine. You would need to limit the slip of the front wheels or the front/rear diff would allow the front wheels to increase rotation speed without limit.
     
  12. Jul 8, 2016 #11

    sophiecentaur

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    I just re-read this. Is the answer that you're looking for that the torque from the front wheels is in the direction to push the CM downwards and not upwards, as with the front wheels?
     
  13. Jul 8, 2016 #12
    I originally thought it has to do with what direction the car accelerates, but then again the two posts from you and svein got me into thinking that where the power from the engine is applied (to front or rear wheels) is the fact that plays the major role.

    I think in order to see the back wheels raising, we got to have the car moving in reverse and the power from the engine to the front wheels...
     
  14. Jul 8, 2016 #13

    A.T.

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    Regardless if the back or the front wheels are powered, you get the same torque direction. But the direction of rotation to lift the front wheels is opposite to lifting the back wheels.
     
  15. Jul 8, 2016 #14

    sophiecentaur

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    I'm not sure if you're agreeing with me or not. But the sense of the torque will produce different directions of force, depending on where you are taking moments from.
    It makes you wonder why dragsters have their driving wheels at the back . . . . ? Maximum weight on the driving wheels, I suppose.
     
  16. Jul 8, 2016 #15

    jbriggs444

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    When you apply the brakes in a forward-moving vehicle, rear wheels lift somewhat. That is why front brakes are so much more important for stopping than rear brakes. Brake hard enough (and with high enough traction and center of gravity) and the rear wheels can actually lift off the ground.

    https://en.wikipedia.org/wiki/Stoppie
     
  17. Jul 8, 2016 #16
    I was thinking of that too, during braking the friction force reverses direction, about same thing as going with reverse. The only problem with reverse is that the power had to be in the front wheels, and the acceleration big enough but that last thing we know it cant be true, reverse doesn't give big acceleration.
     
  18. Jul 8, 2016 #17

    jbriggs444

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    In a typical front wheel drive passenger car I would expect reverse to deliver a peak acceleration greater than forward.
     
  19. Jul 9, 2016 #18

    A.T.

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    Yes, the intial accelarion should be greatest in reverse. But a typical car still doesn't have enough torque to lift either wheels by it's own acceleration.
     
  20. Jul 9, 2016 #19
    I am not getting you both AT and jbriggss , doesn't depend solely on the design of the gearbox whether the acceleration would be bigger in reverse than in 1st gear?
     
  21. Jul 9, 2016 #20

    jbriggs444

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    That depends on whether peak thrust is limited by engine power and gearing or by traction. If the car has enough power to spin its wheels in first gear (and in reverse which has similar gearing) then its acceleration is traction-limited instead.

    You will get the best traction when the acceleration tends to raise the non-driving wheels from the ground. Weight is transferred to the driving wheels, giving them more traction.
     
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